1. Background of the Invention
The present invention relates to an apparatus according to the preamble of claim 1 for growing thin films on a substrate, in which apparatus the substrate is subjected to alternately repeated surface reactions of vapor-phase reactants for the purpose of forming a solid-state thin film on the substrate through said surface reactions.
The apparatus comprises a reaction chamber pack into which the substrate can be placed, at least two reactant sources from which the reactants used in the thin-film growth process can be fed in the form of vapor-phase pulses into the reaction chamber pack, reactant inflow channels suited for connecting the reactant sources to the reaction chamber pack, and outflow channels connected to the reaction chamber pack suited for removing the gaseous reaction products of the growth process and the excess reactants.
2. Related Art
Conventionally, thin-films are grown using vacuum evaporation deposition, the Molecular Beam Epitaxy (MBE) and other vacuum deposition methods, different variants of the Chemical Vapor Deposition (CVD) method (including low-pressure and metal-organic CVD and plasma-enhanced CVD), or alternatively, the above-described deposition method of alternately repeated surface reactions called the Atomic Layer Epitaxy method, or in short, ALE. In the MBE and CVD methods, the growth-rate-affecting process variables also include the concentrations of the starting material inflows. To achieve a uniform thickness of the layers deposited by the first category of conventional methods, the concentrations and reactivities of starting materials must be carefully kept constant all over the substrate area. If the starting materials are allowed to mix with each other prior to reaching the substrate surface as is the case in the CVD method, for instance, a chance of their mutual reaction arises. Then, the risk of microparticle formation already within the inflow channels ol the gaseous reactants is imminent. Such microparticles have a deteriorating effect on the quality of the thin film growth. Therefore, the possibility of premature reactions in MBE and CVD reactors is avoided by heating the starting materials not earlier than at the substrate surfaces. In addition to heating, the desired reaction can be initiated using, e.g., a plasma or other similar activating means.
In the MBE and CVD processes, the growth of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate. By contrast, the ALE process is based on allowing the substrate surface qualities rather than the starting material concentrations or flow variables to control the deposition rate. The only prerequisite in the ALE process is that the starting material is available in sufficient concentration for film formation on all sides of the substrate.
The ALE method is described in the FI patent publications 52,359 and 57,975 and in the U.S. Pat. Nos. 4,058,430 and 4,389,973, in which also some apparatus embodiments suited to implement this method are disclosed. Equipment constructions for growing thin films are also to be found in the following publications: Thin Solid Films, 225 (1993), pp. 96-98, Material Science Reports 4(7) (1989), p. 261, and Tyhjiotekniikka Finnish publication for vacuum techniques), ISBN 951-794-422-5, pp. 253-261.
In the ALE growth method, atoms or molecules are arranged to continuously sweep over the substrates thus impinging on their surface so that a fully saturated molecular layer is formed thereon. According to the conventional techniques known from the FI patent publication No. 57,975, the saturation step is followed by an inert gas pulse forming a diffusion barrier which sweeps away the excess starting material and the gaseous reaction products from above the substrate. The successive pulses of starting materials and diffusion barriers of inert gas separating the former accomplish the growth of the thin film at a rate controlled by the surface chemistry properties of the different materials. Such a reactor is called a "traveling-wave" reactor. For the function of the process it is irrelevant whether the gases or the substrates are moved, but rather, the different starting materials of the successive reaction steps shall be separated from each other and arranged to flush the substrate sequentially.
When a hot-wall reactor is used, the wall is generally heated by mounting the heater elements and the insulation about a pressure-tight shell. Due to the elevated temperatures employed, inorganic fiber materials or bricks must be used as the insulation materials. The handling and mounting of such insulation release dust in the process equipment location. This poses a conflict with the conventional placement of deposition equipment in a clean room environment due to the risk of film growth disorders caused by dust particles becoming entrapped from the atmospheric air into the film being deposited.
Further, heating of the pressure shell of the deposition equipment also curtails the selection of usable materials fulfilling the requirements of resisting stresses from pressure and heat and being inert at said temperatures with respect to the chemicals occurring in the reactions. Conventional ALE reactors have the pressure shell made of borosilicate or quartz glass and stainless steel. The frameworks of the reactors have an elongated shape and the pressure shells contained therein are heated.
Typically, due to the poor heat resistance of the elastomer seals employed, the deposition equipment is designed with an elongated shape having the operation inlets/outlets placed at the ends to provide a sufficient thermal gradient between the hot parts and the seals of the equipment. In large apparatuses, the heated masses and required heating powers become substantial, and thermal gradients which are difficult to eliminate are formed between the different parts of the equipment. This makes the equipment dimensions larger, and while the different sources of reactants must be thermally isolated from each other by placing the sources enclosed in separate heater casings with an isolating air gap between the casings, the entire apparatus assumes an elongated shape with separate, outward protruding source tubes. The sources may also be placed so that the reaction space is followed by the sources arranged in-line. In such a construction, the highest-temperature reaction space is situated at the other end of the apparatus and the sources are placed inside the zones of the elongated reactor so that the source requiring the lowest temperature is situated in the zone of lowest temperature farthest from the reaction space. A problem herein arises from the control of temperature gradient (because the source at the reaction space end of the source is operated at a higher temperature than at the other end of the source) and from the increased length of the apparatus.
A tubular construction may also require the use of concealed joints which may be inaccessible to the operator thus preventing him from positively verifying successful completion of connections to the substrate holder (this is because the channels of the substrate holder are connected to the other piping within the hot reaction tube).
Due to poor performance of any bearings in a hot environment, the substrate holder must be elevated to the reaction space by a slide or lift mechanism. This results in formation of abraded material dust on the sliding surfaces and scratching of glass substrates due to possible jerks during the movement.
The above-mentioned drawbacks are accentuated in the processing of larger-area substrates, whereby the size and control of the equipment and its components become more complicated.
It is an object of the present invention to overcome the drawbacks of conventional technology and to provide an entirely novel apparatus for growing thin films using the ALE process. It is a particular object of the invention to solve the problems associated with a heated pressure shell without compromising the benefits of a hot-wall reactor.